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Why does Mac Mail give me the error "501 syntactically invalid HELO" when trying to send mail?

This error means that you are using mac mail and have a BT connection, where there is a known issue that causes the hostname of the machine to be generated as an invalid name, which our servers won't allow. To correct this you should just need to use the following guides to change the machine name and hostname: 

Make sure to restart your mac once this is changed as sometimes this is required to make the changes stick. If this does not resolve the error, or if you are not using a BT connection but still seeing this error, let our support team know and they will be happy to help.
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